In certain scientific, engineering and medical fields, it is important to provide closed-loop parametric adjustment, control, or compensation of a physical field within an Operational Volume (OV). The adjusted, controlled or compensated field may exist within a sub-space of the OV defined herein as the Protected Volume (PV), where the PV is partially shielded from the adjusting, controlling or compensating field generating element(s) and also from the field parametric feedback sensor by an intervening, Interacting Medium (IM). An IM is an interval or region of material, of any phase or combination of phases, i.e., solid, liquid, gas or plasma, that physically interacts with site interfering and compensating, or alternatively, controlled fields.
It is most advantageous to compensate for any arbitrary anisotropic, proximity-dependent or frequency-domain effect of the IM, which results in a capability to improve accuracy in the spatial and temporal response of the control or compensation field within the PV.
To illustrate, counteracting site-specific interfering magnetic fields induced within the isocenter (i.e., interior or PV), of the magnet associated with a magnetic resonance imaging (MRI) instrument would be most helpful in operating such MRI devices at the highest possible resolution in such locations. Historically, certain techniques have been employed in problematic MRI installations to achieve isocenter field control utilizing intentionally superimposed extraneous magnetic fields for compensation (subtractive nulling) of extraneous interfering magnetic fields. At MRI sites, the relative coupling behavior of the interfering and the compensating magnetic fields into the MRI magnet isocenter can differ substantially from the relative coupling behavior of the interfering and the compensating magnetic fields into a field sensor (probe) located outside of the magnet windings, or windings and yoke. This effect is mainly due to the differing interactions of the MRI magnet with respect to the compensation coil and geometry of the extraneous interfering source(s).
In general, the MRI's magnet structure consisting of windings (in the case of a superconducting magnet) or windings and yoke (for a resistive, or nonsuperconducting magnet) may be considered an IM that tends to differentially intercept and therefore introduce a differential sensitivity to the compensating field from the Active Compensation Shielding (ACS) driven coils that are in close proximity compared with the interfering field from external sources located at significantly greater distances. With respect to the unity ratio of these fields at the probe, as established by a conventional negative-feedback ACS topology, the MRI isocenter will appear to be either under- or over-compensated. In the former case the residual field component is in-phase and proportional to the interfering field; in the latter, more common case the apparent polarity of the interfering magnetic field component is inverted by the action of the active compensation system. In either case, the finite limitation on interfering field reduction within the isocenter, i.e., PV, constitutes a fundamental and serious restriction on the effectiveness of the ACS when the probe is located at a distance from the isocenter.
High static fields within and surrounding most MRI instruments preclude locating the feedback sensor within or immediately adjacent to the PV. Consequently, the ability to accurately control or cancel extraneous fields within such magnets has previously been limited to standard closed-loop negative feedback approaches that provide only marginally accurate approximation within the critical MRI isocenter region even though the field cancellation behavior at the actual sensor location may be nearly ideal.
When the sensor must be located outside of the PV, a secondary spatial consideration is compensation of possible differential error due to an interfering field gradient along the baseline between the sensor, located outside of the PV, and the isocenter within the PV. Differential error occurs because the compensation field is nominally constant throughout the volume for optimum system performance, therefore the residual field, which is equal to the difference between the interfering field from a given source and the compensating field, increases with distance from the field sensor when a field gradient along the baseline is present. In this situation, also, the canonic ACS approach cannot achieve ideal interfering field compensation.
In addition to the spatial considerations described above, significant temporal degradation of control or compensation accuracy for a canonic ACS feedback system can occur in applications where a physically Interacting Medium exists between the OV, where the sensor is located, and the PV where ideal field control or compensation is desired. For an MRI instrument, the physically Interacting Medium responsible for temporal alteration is represented by, depending on the installation, either the ferromagnetic yolk and conductive coils of a resistive electromagnet or the conductive coils of a superconducting magnet. Because the magnetic field sensor must be located outside of the IM-shielded MRI magnet isocenter, a conventional overall control or compensating system as described in the prior art has no “awareness” of the IM effect on traversing fields and, consequently, cannot compensate for frequency-dependent or direction-dependent interactive effects of the MRI's IM region.
The deleterious effects of an MRI's IM on the control or compensating system accuracy are thus twofold. Principally, the IM modifies the magnetic field traversing it in amplitude and phase, with the modification coefficients varying as a function of source direction and frequency. In a significant fraction of installations subject to problematic magnetic field interference, the MRI IM exhibits sufficient time-dependent field interaction that frequency-specific correction is required in order to achieve adequate compensation of the interfering field within the magnet isocenter or, alternatively, programmable control of a supplemental field within the isocenter. To achieve satisfactory parametric control of the residual field within the PV, it is therefore necessary to incorporate compensation for such temporal IM effects into the MRI active-compensation feedback signal processing system.
It would therefore be desirable to create a control or compensating system for such MRI applications, or other similar applications involving physical fields to be controlled or compensated, where exacting control or compensation of such fields may be provided within the PV, even with an appreciable distance between a sensor located in the OV and external to the PV.
A fundamental principle of a requisite ACR implementation is that spatial and temporal ratiometric offset control or compensation can be implemented by means of a rigorously predictable set of parametric coefficients that theoretically match and cancel any gradient or dynamic IM effects on the superimposed fields within the PV.
For a field-compensation situation where an IM exists that physically modifies the fields operating within a system PV, it is also probable that the IM will affect the interfering and compensating fields differently. Interaction disparity of this type can be due, for example, to relative proximity of the interfering source and compensating means, or to IM anisotropy with respect to the interfering source and compensating field dominant axes.
A second fundamental principle of the ACR invention is that it permits direct compensation for such interaction differences by employing an expanded set of rigorously predictable parametric coefficients that define the required compensation. These coefficients may be determined by analysis of measured data or by empirical tuning in situ. Once set, the coefficients do not usually require readjustment, as they are dependent only on invariant characteristics of the IM itself.
In the example embodiment, one may consider the MRI magnet assembly to be the IM. The magnet may be permanent, resistive or superconducting. Each type of magnet affects the residual fields from the interfering and compensating magnetic field sources in a slightly different way. For permanent and resistive magnets, the magnet yoke permeability and geometric flux path vary anisotropically with source direction, leading to intensity differences in the isocenter and errors in field cancellation. In resistive magnets, shunted windings can also contribute frequency-dependent magnitude and phase variations that magnify the cancellation error. Superconducting magnets, while largely self-shielding per Lenz's Law, in practice allow some ingress of leakage flux into the magnet isocenter. The resulting interfering field usually exhibits time-delayed field variations due to induced redistribution of persistent currents in the magnet windings.
For any of these magnet types, residual flux demonstrating pronounced frequency-dependent magnitude and phase modification can be found in the magnet isocenter at an operationally significant level when sufficiently strong external magnetic fields are present. When an external compensating field is applied by a closed-loop feedback system utilizing a field sensor external to the isocenter, the residual magnetic field within the isocenter is thus described by the frequency- and direction-dependent difference between the modified interfering field and the modified compensation, or cancelling, field. Therefore, when addressing problematic MRI site magnetic interference, frequency-dependent differential compensation uniquely provided by the ACR invention is necessary to achieve a high level of interfering signal cancellation.
Application of the Adjustable Compensation Ratio feature of the instant invention is realized by a secondary feedback means wherein parametric information that describes the magnet's self-shielding transfer characteristic is incorporated into the overall ACR feedback loop. This information enables arbitrarily exact cancellation of the fields within the magnet solely as a function of the inner feedback loop parametric accuracy. These parameters are usually determined empirically on site but, alternatively, can be derived analytically from suitable measurements describing a specific magnet's field translation characteristic. Importantly, once the frequency- and direction-dependent parameters have been set within the secondary feedback loop, the system parametric sensitivity to environmental changes external to the magnet is low due to the large open-loop gain of the overall ACR system feedback loop. Consequently, even though the magnetic field sensor is located outside the magnet proper, a high initial attenuation factor within the isocenter PV can be maintained, without the need for periodic system “tuning” adjustments, regardless of moderate changes in the MRI system ferromagnetic environment.
Additional embodiments for other types of influential fields include shielding or controlling electrostatic fields, such as within an isolating dielectric medium, shielding or controlling electromagnetic energy irradiating an OV, such as within a partially conductive plasma sheath and shielding or controlling acoustical energy within an OV that may be surrounded by a medium physically interacting with such energy.
In other embodiments, Active Cancellation Systems (ACS) can be provided similar to the magnetic field case, or for control or adjustment of the parameters as needed to minimize or adjust either the absolute magnitude of, or variation of fields within volumes semi-shielded from the controlling influence.
Developed for improved interfering field attenuation with respect to previous magnetic active-shielding technology, the instant invention finds immediate utility in that area. There, as described above, it adds the capability to deeply null residual magnetic field interference over an arbitrary volume that is separate, and shielded to some degree from the active shielding system's magnetic sensor and compensating field induction coils. This specific application notwithstanding, the invention described by this disclosure is applicable to any closed-loop negative-feedback system intended to mitigate and/or control the influence of external field sources within a Protected Volume (PV), when the sensing means is external to the PV.